When flowing lava on the surface engulfs trees, the
lava may cool before the tree has completely burned away, and leave
an imprint of the tree in the lava. Basically this is a type of fossil
known as a cast. Tree molds are more commonly found within surface lava
flows, but occasionally they are seen underground as in the example
below. There are also lava tree mold caves, which consist entirely of
tree molds which may connect together. One such is a public-access cave
in Mount Saint Helens National Monument, near Ape Cave.